Parametric loudspeaker systems are known that employ ultrasonic transducers for projecting ultrasonic carrier signals modulated with audio signals through the air for subsequent reproduction of the audio signals along a selected path of projection. A conventional parametric loudspeaker system can include a modulator for modulating an ultrasonic carrier signal with an audio signal, at least one driver amplifier for amplifying the modulated ultrasonic carrier signal, and one or more ultrasonic transducers for directing the amplified, modulated ultrasonic carrier signal through the air along the selected projection path. For example, each ultrasonic transducer can be a membrane transducer, such as an electrostatic transducer or a membrane-type piezoelectric transducer. Due to the non-linear propagation characteristics of the air, the modulated ultrasonic carrier signal is demodulated as it passes through the air, thereby reproducing the audio signal along the selected projection path.
In such a conventional parametric loudspeaker system, the modulation depth of the ultrasonic carrier signal has traditionally been allowed to remain relatively small when low-level audio signals are to be reproduced. Because the ultrasonic carrier signal itself is typically maintained at a high level, such low-level audio signals would cause a slight modulation of the ultrasonic carrier signal, while higher-level audio signals would cause a deeper modulation of the ultrasonic carrier signal. Such a modulation approach has drawbacks, however, in that it can adversely affect the efficiency of the parametric loudspeaker system, allowing the system to generate high ultrasonic signal levels even in the absence of audible sound. Increasing the modulation depth of the ultrasonic carrier signal when modulated with low-level audio signals may enhance the overall efficiency of the parametric loudspeaker system. However, this further modulation approach also has drawbacks in that it can increase the bandwidth of the ultrasonic signal, potentially increasing audible distortion upon reproduction of the audio signal.
It would therefore be desirable to have improved modulation systems and methods for use in parametric loudspeaker systems that can avoid at least some of the drawbacks of conventional parametric loudspeaker systems.